how much of a difference should equipment make?

I spent the last few months looking for a good sounding 2 channel setup and this is what I ended up with:

McIntosh C50 Preamp
McIntosh MC275 Amp (VI)
SF Cremona M
Audioquest King Cobra XLR
Audioquest Carbon USB
Audioquest Rockefeller SW

Let me just get this out of the way and say it up front.. While the speakers (and their placement in my room) seemed to make a huge difference, the amp, preamp, and cables do not seem to make any discernable difference to me or my wife.

I feel like I have purchased the C50/MC275 due to a combination of the integrated DAC, aesthetics, heritage, level of support, and resale value.

We compared several options at home and in high end audio shops.. For example, in one store, I got to listen to the Cremona M's through a pair of MC1.2kw+C2300+AyreQB9 and then immediately following that, the C50 paired with a single MC275. I really wanted to hear a difference, but nothing. In the same store, I was also able to listen to a Music Fidelity M6 and luxman 550AX connected to the same speakers.

I brought home a demo of a luxman 550AX integrated and an MA6600 McIntosh integrated and did several days of A/B testing with the Cremona M's and we could not detect a difference at all. I tried to make sure that decibel levels were matched by playing a test tone and using an SPL meter.

I am using Jriver with ASIO with a variety of music off of hdtracks.. the music is being sent to the DAC in the C50 via audioquest carbon USB cable in bit perfect asynchronous mode.

I checked the MC275 manual and McIntosh recommends 12AWG wire. When I had a pair of audioquest type 4 connected (which was only 16 AWG), we did detect a small improvement when moving to the 12 gauge Rockeller wires. I did not test any cheaper speaker wire. This was the only time that I heard something other than the speakers make an audible difference.

I was also able to compare a peachtree iDAC and Ayre DAC at my home and I did not detect much (if any) difference between the DAC built in to the C50. I do have the latest firmware.

I heard huge differences when cross shopping speakers and doing A/B tests in stores.. for example, B&W 802D seemed to have much more 'oomph' on the low end, but lacked some midrange and high vocal clarity. It felt a bit like voices were not 'tight' compared to the Sonus Faber line. I listened to a pair of SF Elipsa and they sounded similar to the Cremona M but a bit more 'full'. As if the depth of the sound stage increased and also the bass had a bit more impact.

Although I do not hear any difference in preamp/amp/cabling, it does not mean that a difference does not exist. I am not trying to insult anyone here.. it may just be that the combinations of equipment that I have tried are still somehow 'lacking'.

My questions are as follows....:

1) Is it normal to not hear much (or any??) difference when changing out preamps or amps assuming the components in question are within similar specs?

2) Has anyone else purchased relatively expensive amp/preamp combinations even when you believe the components are not making a difference in the sound quality?

3) Placement and angle of my speakers in my room make almost as much of a difference as the brand and model of the speaker - at least to me. Anyone else have this experience?

4) What am I doing wrong that is leading to not hearing a difference?

I do not believe my ears are at fault.. My wife and friends were with me throughout most of my testing and they came to the same conclusions.

In any event, my setup at home sounds great to me and rivals (again, to my ear) most everything I have heard in audio stores.

Thanks,